Process for preparing chrome tanning material



Patented Jan. I 1952 PROCESS FOR PREPARING CHROME TANNING MATERIAL Robert P. Follett, Needham, and Wallace J. Murray, Dedham, Mass, assignors to Arthur D. Little, Inc., Cambridge, Mass, a corporation of Massachusetts No Drawing. Application August 10, 1950, Serial No. 178,761

Claims. (01. 260 406) This invention relates to a process for preparing a chrome tanning material from hexavalent chromium compounds concurrently with the production of useful by-products of the reaction which represent intermediate materials leading to plasticizer compositions, and more particularly relates to an improvement on L. H. B. Hervey Patent No. 2,178,874, dated November '7, 1939.

As pointed out in the Hervey patent, chromium is normally recovered from natural sources in its hexavalent state. Such hexavalent chromium, however, is not available for use as a tanning material since it must be in its trivalent state for this purpose. It has been the practice in the art prior to Hervey, and continuing down to the present time, to reduce hexavalent chromium with' any convenient reducing agent without recovering anything from the reaction other than the desired salts of trivalent chromium whereby the oxidizing power of the hexavalent chromium is wasted. Thus, it has been the practice to employ glucose for this purpose, which is largely oxidized to carbon dioxide and water.

The aforesaid Hervey patent discloses the conversion of a compound of hexavalent chromium, such as a dichromate, to chromic acid by treatment with a non-oxidizing acid of sufficient strength to convert the dichromate into chromic acid and maintain the same in that form, a suitable acid being sulfuric acid. Following this step, there is taught the reduction of the chromic acid to trivalent chromium compounds with an unsaturated organic compound, such as an unsaturated fatty acid, whereby oxidative attack of the acid at the double bond takes place and saturated monoand dicarboxylic acids of lower molecular weight are produced. Moreover, the Hervey patent is directed to a means of separating the said saturated acids from the reduced chromium solution, whereby the reduced chromium solution is recovered for subsequent application in the tanning industry and the saturated acids are recovered for various uses in the arts.

The proposal of Hervey, as it appears in his patent, is characterized by a serious practical difficultyapparently realized by Hervey since he hints at the same, but in any case rendering the Hervey process of doubtful commercial significanoe in view of the gravity of the problem. The problem arises from the following set of circumstances:

The useful and therefore sought material for tanning purposes is a basic chromic salt, such as basic chromic sulfate, Cr(OH)SO4. In order to obtain such compound, the ratio of sulfuric acid to dichromate employed to form the chromic acid prior to reduction must be specifically adjusted, and as Hervey points out, is desirably in the mol ratio of three parts of acid to one part of dichromate. Although it is specifically pointed out by Hervey that this is the optimum ratio toobtain the basic chromic sulfate for tanning purposes, nevertheless Hervey teaches (page 3, at lines 20-25) that the use of a different and higher ratio of acid to dichromate is preferred. There is no explanation in Hervey of this anomalous statement, but there is an excellent reason why Hervey preferred the higherratio of acid to dichromate. I

When employing the ratio of three mol parts of acid to one mol part of dichromate, the conversion to chromic acid occurs quantitatively and without any technical difliculties. However, upon reducing the chromic acid to trivalent chromium compounds with an unsaturated organic material, such as an unsaturated fatty acid, the difficulties of separation of the ultimate products, i. e., the basic trivalent chromium compounds and the lower molecular weight saturated mono-, and dicarboxylic acids, are very substantia1. In the presence of this amount of acid, an emulsion forms in the reaction vessel which, upon the conclusion of the reaction, is more or less of the order of oatmeal in consistency. This emulsion defies ordinary means of breaking the same to separate the desired basic chromic compounds from the desired acids produced in the reaction.

As may be noted from the Hervey patent, particularly col. 1, on page 3, Hervey found this problem so profound that his solution amounted to an avoidance rather than a meeting of the problem directly Thus, Hervey preferred to employ a 4: 1 ratio of acid to dichromate in forming his chromic acid whereby, upon subsequent reaction of the chromic acid with the unsaturated fatty acid or the like, no emulsion formed in the reaction mass and the products were satisfactorily separable; However, the disadvantage of this procedure, which is pointed to without comment by Hervey, is that instead of ob taining basic chromic sulfate desired for tanning purposes, chromic sulfate is obtained which. must be further treated in order to obtain a tanning compound.

The gravity of the separation problem, when employing a 3:1 ratio of acid to dichromate, is clearly demonstrated by Herveys recommendation that the 4:1 ratio be used, followed by modification of the chromic compound obtained either by addition of caustic or by addition of further dichromate. Caustic addition is, for all of its recommendation, most undesirable since, although it produces the desired basic chromic sulfate for tanning purposes, it introduces by addition of sodium ions to the solution. an undesired amount of sodium sulfate in the final tanning compound, whereby the tanner cannot be provided with the specific composition which he desires for his purpose. the same being too concentrated in sodium sulfate.

The addition of more dichromate to the system is no better. in that it then requires further reduction which may not be accomplished in accordance with Herveys. teaching with unsaturated organic compounds, but on the c ntrary must be accomplished by a dit on of glucose, the prior art reducing a ent which Hervey so ht to avoid in order to obtain. upon further reduction, the basic chromic sulfate desired. It will be appreciated that b th of these expedients for avoiding the d flicu t separat on problem of the 3:1 acid ratio mate ial are cumbersome and unwieldly and actually on av id the roblem of the prior art of employing the ox diz ng value of dichromate in the pro uct on of basic ch omic tanning compounds, rather than meeting it head-on and solving it.

The present invention has as its principal object th solut on of the problem posed by He vey of employ ng the 3:1 acid to dichromate ratio in formin the chromic acid, and subseouently treating this chromic acid composition w th unsaturate organic material, such as unsatu ated fatty aci s, to obtain a combined product of basic ch omic sulfate and saturated lower molecular weight acids, and more particularly is directed to the specific separation p oblem which Hervey found insoluble, whereby the clumsy expedients of addition of caustic or addition of further dichromate and further reduction with glucose can be avoided.

In accordance with the present invention, it has been found that upon the treatment of the 3:1 ratio chromic acid mix with suitable fatty acids. such as those disclosed on page 1 and elsewhere of the Hervey patent, which are specifically incorpo ated here by reference, an emulsified material is obtained wh ch in fact does defy ordinary m ans of separation. However, in accordance with the present invention this emulsion may suitably be treated by relatively sim le means not heretofore appreciated to be applicable in this environment, to separate the same and obtain the valuable products thereof in relatively high purity and in conditi n for use in the arts. Thus, in accordance with the present invention it has now been found that if the emulsified reaction product of the unsaturated organic compound and the chromic acid prepared by combination of dichromate and sulfuric acid in a mol ratio of 1:3 is suitably dried to remove therefrom substantially all of the water associated with the reaction product, then by extraction with organic solvents the separation of organic from inorganic products becomes relatively simple and straightforward and both products are recoverable in substantially quantitative yield.

It will be appreciated by those skilled in the art in view of the disclosure above that this drying may be accomplished by various convenient means. One means of drying which is preferred is the azeotropic distillation of the water from the emulsified reaction mixture with toluene, 2-ethyl hexanol, or any other solvent suitable for the purpose. Upon the completion of the azeotropic distillation with removal of substantially all of the water, extraction of the organics remaining in the reaction mass may be accomplished by any convenient extracting agent, such as Z-ethyl hexanol, toluene, ethyl ether, or any other convenient solvent for the organics in the mixture. This procedure is in direct and sharp contrast to the experience had in attempting to extract the undried reaction mixture where the addition of the same extracing agents as those mentioned above will in fact intensify the emulsion rather than effect any separation whatsoever.

A subsidiary but very significant feature of the invention lies in the use of 2-ethyl hexonal, as an extracting agent, since especially in the case of oxidation of red oil, more particularly disclosed in Hervey Patent No. 2,178,874, at the top of col. 2, page 2, and specifically incorporated here by reference, the products of the reaction comprise principally azelaic, pelargonic and caproic acids which, when combined with 2- ethyl hexanol, may suitably undergo esterification under conditions well known in the art and not forming a part of this invention, to yield materials available for use as plasticizers or for the preparation of plasticizers for various types of thermoplastic materials, especially the vinyl plastics, particularly vinyl chloride.

It is to be understood that the present invention is by no means directed merely to the azeotropic distillation method of drying the reaction mass under discussion, but on the contrary contemplates all possible methods of drying the mixture, others which might be suggested being spray or drum-drying of the mixture to remove water therefrom, following by extraction with the solvents suggested above. As long as the heat applied to the material to dry the same is insuflicient to cause any degradation of the organic products present, the extraction subsequent to drying has been found to be straightforward and to ossess none of the problems actually met by Hervey and solved by him in the round-about fashion discussed above.

In summary therefore, the present invention is directed to an improvement in the method disclosed by Hervey, which improvement includes the obtaining of a material available directly and without further modification for chrome tanning purposes, i. e., a basic chromic salt, particularly basic chromic sulfate, by reduction of hexavalent chromium compounds with unsaturated organic materials, such as unsaturated fatty acids, to obtain simultaneously valuable by-products in the form of saturated lower molecular weight mono, and dicarboxylic fatty acids, a feature of the invention being the extraction of this mixture after drying with 2-ethy1 hexanol, whereby a mixture is provided available for convenient esterification to obtain plasticizers for plastics of the thermoplastic type, such as vinyl compounds and the like.

In order that those skilled in the art may more specifically understand the purpose of this invention and the method in which it may be put into practice, the following specific example is offered:

Example A solution of 200 gms. of sodium dichromate dihydrate in 250 mls. of water is treated with 111 mls. of sulfuric acid (205 gms. at 96.5% assay) equivalent to a 3:1 acid to dichromate ratio. 194

mls. of this solution are placed in a flask equipped with a magnetic stirrer, thermometer, cold finger and reflux condenser. 50 mls. (44.2 gms.) of oleic acid are added dropwise to the solution at 80 C. over a period of 3 minutes, during which time the temperature rises from 80 to 112 C. and clear evidence of reaction is observed. At the end 'of one and one-half hours at reflux with stirring, a sample is tested for completeness of oxidation, and oxidation being found incomplete, refluxing is continued for one additional hour. At the end of this time, the reduction still being incomplete, 25 mls. of oleic acid are added and the mixture is refluxed for 5 hours, at which time reduction is complete.

At this point the reaction mass is a gummy, emulsified system which resists separation by extraction with Z-ethyl hexanol, ethyl ether, carbon tetrachloride, and the like. Separation is possible when additional mineral acid is added, but appears impossible by straight extraction without additional acid. Moreover, addition of more mineral acid brings about the same disadvantages as those resulting from the Hervey procedure.

One hundred mls. of toluene and three hundred mls. of 2-ethylhexanol are thereupon added to the reaction mass and this mixture is heated to boiling under a trapped reflux, the Water driven off being caught in the trap and the remainder of the condensate returned to the material being refluxed. After seven hours of distillation all the water is removed; the distillation is then stopped and the remaining mass filtered on sintered glass. The filtrate contains the bulk of the organic acids in solution. The filter cake is then washed with Z-ethylhexanol to recover additional organic material remaining in the cake. The washings and filtrate are united and 5 cc. of sulfuric acid added. This mixture is heated under a trapped reflux to esteriiy the organic acids and the Z-ethylhexanol. The toluene serves to carry to the trap the water produced during this esterification. After esterification the mixture is washed with water to remove the sulfuric acid, and then distilled. The toluene is first recovered, followed by excess Z-ethylhexanol. The mixed esters themselves may be separated by distillation into the Z-ethylhexyl esters of caproic, pelargonic and azelaic acids respectively, although this is not necessary.

Di-2-ethylhexyl azelate per se is useful as a plasticizer for resins, particularly polyvinyl chloride, and the 2-ethylhexyl esters of caproic and pelargonic acids are suitable as intermediates for preparing plasticizers for resins.

The remainder from the extraction is found to consist largely of basic chromium sulfate, the same being applicable directly in chrome tanning operations.

While there has been described an embodiment of the invention, the method described is not intended to be understood as limiting the scope of the invention as it is realized that changes therewithin are possible and it is further intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever form. its principle may be utilized.

W hat is claimed is:

1. The process of simultaneously preparing a tanning material and valuable by-product organic acids which includes contacting a hexavalent chromium compound with sulfuric acid in the ratio of one mol of chromium compound to about three mole of acid, and treating the resultant product with unsaturated organic acids capable of yielding primary oxidation products stable in the environment of oxidation, simultaneously and mutually oxidizing and reducing said organic acids and said bichromate to yield basic chromic sulfate, drying the reaction mass of substantially all of the water contained therein, and separating said basic chromic sulfate from said cried product.

2. The method of claim 1 in which organic acids of lower molecular Weight are produced in the mutual oxidizing-reducing reaction and said basic chromic sulrate is separated rrom said dried product by extraction with an organic sol.ent.

3. The method of claim 2 in which the said lower molecular weight organic acids are extracted from said dried product with Z-ethyl hexanol and subsequently in the presence of an acid catalyst, said acids and said hexanol are reacted to form their esteriflcation products.

4. The method of claim 1 in which the hexavalent chromium compound is sodium dichromate.

5. The method of claim 1 in which the unsaturated organic acids comprise a mixture of organic acids.

6. The method of claim 1 in which the unsaturated fatty acids comprise red oil.

7. The method of claim 1 in which the unsaturated fatty acids comprise oleic acid.

8. The method of claim 1 in which the drying of said reaction product is accomplished by the addition of an organic solvent and azeotropic distillation from said mixture.

9. The method of claim 1 in which the drying is accomplished by azeotropic distillation with toluene.

10. The method of claim 1 in which the drying is accomplished by spray-drying of the reaction mass.

ROBERT P. FOLLETT. WALLACE J. MURRAY.

No references cited. 

1. THE PROCESS FOR SIMULTANEOUSLY PREPARING A TANNING MATERIAL AND VALUABLE BY-PRODUCT ORGANIC ACIDS WHICH INCLUDES CONTACTING A HEXAVALENT CHROMIUM COMPOUND WITH SULFURIC ACID IN THE RATIO OF ONE MOL OF CHROMIUM COMPOUND TO ABOUT THREE MOLS OF ACID, AND TREATING THE RESULTANT PRODUCT WITH UNSATURATED ORGANIC ACIDS CAPABLE OF YIELDING PRIMARY OXIDATION PRODUCTS STABLE IN THE ENVIORNMENT OF OXIDATION, SIMULTANEOUSLY AND MUTUALLY OXIDIZING AND REDUCING SAID ORGANIC ACIDS AND SAID BICHROMATE TO YIELD BASIC CHROMIC SULFATE, DRYING THE REACTION MASS OF SUBSTANTIALLY ALL OF THE WATER CONTAINED THEREIN, AND SEPARATING SAID BASIC CHROMIC SULFATE FROM SAID DRIED PRODUCT. 